User blog comment:Nymfia/Halloween Event Pokemon/@comment-31800919-20171014003233

The mimikyu one you dont need to have a pikachu in the party, finding a pikachu at night will still trigger the event.

The marshadow one this info is from jamiyjamie's vid the marshadow portal will randomly spawn either at route 9,10,11 or 12 there 3 other spots one being cosmeos valley but he is keeping the other 2 a secret.